1. Field of the Invention
The present invention relates to an engine intake system, and more particularly to an engine intake system wherein a compression wave produced in the intake passage is utilized to increase the intake charge.
2. Description of the Prior Art
Hithertofore, it has been proposed to accomplish a high engine output by increasing the intake charge through a utilization of the inertia of the intake flow, the intake pressure resonance effect or the intake flow dynamic effect. In case where an increase in the intake charge is sought through a utilization of the pressure wave produced in the intake passage, several problems are encountered. One of such problems is that the pressure wave in the intake passage is weak under a low speed engine operation so that an adequate improvement cannot be achieved. Further, it is difficult to obtain improved results in increasing the intake charge through a broad engine operating range due to the fact that the compression wave propagates at the sonic speed but the engine speed changes in a wide range.
In view of the foregoing problems, Japanese patent application No. 54-12817 filed on Feb. 8, 1979 and disclosed for public inspection on Aug. 16, 1980 under the disclosure No. 55-107018 proposes an engine intake system including an intake air reservoir provided in the intake passage, and a control valve located in the intake passage between the reservoir and the intake valve. The control valve includes a valve disc which is rotated at a speed which is the same as or one-half of the speed of the engine crankshaft so that the communication between the reservoir and the cylinder is controlled is accordance with the engine speed.
In general, the engine speed N in which the intake charge can effectively increase through a utilization of the compression wave can be represented by the following formula (1). EQU N=.theta.e.nu./6 (1)
wherein: .theta.e is the effective valve opening period in degree, and .nu. is the natural frequency in Hz of the intake system while the intake valve is open.
The effective valve opening period is defined as the period wherein the intake gas is actually drawn into the combustion chamber and does not exactly conform to the actual valve opening period. According to the teachings by the aforementioned Japanese patent application, the opening period of the control valve is controlled within the opening period of the intake valve so that the effective valve opening period corresponds to the opening period of the control valve. The Japanese patent application teaches to control the opening timing of the control valve in accordance with the engine speed to thereby change the timing in which the pressure wave is produced. The proposed intake system is therefore effective to a certain extent to expand the engine speed range in which the compression wave can be utilized to increase the intake charge.
It should however be noted that, in the proposed system, the opening period of the control valve is always within the opening period of the intake valve and the effective valve opening period .theta.e remains unchanged. Further, the natural frequency .nu. of the intake system does not change. Therefore, it is only possible within the opening period of the intake valve to expand the speed range in which the intake charge can be increased. Thus, the proposed system can perform an improvement only to a limited extent.
It should further be noted that the Japanese patent application teaches to retard the opening timing of the control valve in the low speed range so that the pressure wave can be intensified, however, this arrangement will cause an increase in the pumping loss. Thus, there will be an adverse effect on the fuel economy in light road engine operation. A further problem in the proposed system is that the effective valve opening period is determined essentially by the control valve which has the opening period smaller than that of the intake valve, so that the control valve produces an additional resistance to the intake flow. Therefore, it becomes difficult to ensure an adequate intake charge in a high speed, heavy load engine operation.